1. Field of the Invention
The present invention is directed to stabilizing the output pulse energy in a continuously pumped, repetitively cavity-dumped laser.
2. Description of the Related Art
A laser, in its simplest configuration, operates in a “continuous wave” mode, in which the power output is relatively constant over time. For many applications, such as cutting, drilling, and investigation of media properties, it would be beneficial to use a pulsed laser, in which the power output is extremely high over a very short period of time. In such a pulsed laser, the output power is effectively “stored up” in the laser cavity over a period of time, then released in a short pulse. During the duration of the laser pulse, known as the pulse width or pulse duration, the output power of the laser may be many orders of magnitude larger than the continuous wave output power. A preferable method of generating these short pulses is known as mode locking. Mode locked lasers can routinely produce pulses with durations of picoseconds.
It is often desirable to boost the energy contained in these pulses without significantly affecting the pulse durations. A common technique for generating microjoule level pulses without using complex and expensive amplifier schemes is known as “cavity dumping”.
In cavity dumping, the basic idea is to keep the optical losses of the laser cavity as low as possible for some time, so that an intense pulse builds up in the cavity, then to extract this pulse within about one cavity round-trip time using an optical switch, which may be acousto-optic or electro-optic. The switch may be referred to as a “cavity dumper.” Cavity dumping is explained in greater detail in the following paragraphs.
A laser cavity used for cavity dumping is similar to that used for Q-switching, but containing only highly reflecting mirrors (i.e., no partially transmissive output coupler mirror). Output coupling is controlled with the optical switch in the cavity, typically a combination of an electro-optic modulator (EOM) and a polarizer, which is quickly turned on for pulse extraction and then directs the intracavity beam into the output. At times other than during pulse extraction, the light can circulate in the resonator with low losses.
Pulse amplification then occurs as follows. Initially, the modulator is set so that most of the light in the cavity is coupled out of the cavity. In this initial state, the power is below the laser threshold and no lasing occurs. The pump energy in the cavity is stored primarily in the gain medium. Next, the modulator is switched so that the cavity losses are reduced to small parasitic losses. In this switched state, the power in the cavity builds up quickly, typically within a few hundred cavity round-trip times. Finally, the modulator is quickly set so that most of the light is again coupled out of the cavity. In this final stage, the energy in the cavity is extracted in about one round-trip time. The cycle is then repeated.
An advantage of cavity dumping over Q-switching is that prior to extraction of a pulse, the energy in the cavity is stored in the electric field inside the cavity, rather than in the gain medium. As a result, the energy can be extracted much more quickly than for Q-switching, typically, in one round-trip time. This, in turn, allows for high repetition rates for cavity dumping, which is highly desirable for many applications, such as industrial machining, drilling, cutting and surface engineering.
Cavity dumping for ultrashort pulses is mostly used with mode-locked solid state bulk lasers, such as titanium-sapphire lasers or diode-pumped neodymium-doped or ytterbium-doped lasers. The pulse energy from a cavity-dumped, mode-locked laser may typically be about an order of magnitude higher than with an ordinary mode-locked laser (i.e., typically of the order of 1 microjoule), and the pulse repetition rate can be hundreds of kilohertz or even several megahertz or higher.
A potential issue with cavity-dumped lasers is that the pulse energy can undesirably depend on the time between pulses, particularly for fast repetition rates.
One may think of a relaxation time in the cavity, during which the intracavity energy oscillates before reaching a steady-state value. If the repetition rate is sufficiently low, then the system reaches its steady state prior the next cavity dumping event. Therefore, the pulse energies of upcoming output pulses are generally stable over time (because energy of the output pulse is proportional to the intracavity energy at the instant of cavity dumping). In this regime, the intracavity energy at the instant of ejecting the pulse may occur in the relatively flat steady state region. If the repetition rate is changed by a user, or a pause is inserted between pulses, the intracavity energy does not change much. As a result, the energy of the next output pulse is largely unaffected. For these relatively slow repetition rates, the pulse energy is largely independent of time between pulses.
If, however, the repetition rate is high enough so that the intracavity energy does not sufficiently settle to its steady-state value, then the next pulse ejection may occur during the oscillations. Here, the intracavity energy at the instant of ejecting the pulse may occur on a rapidly-changing oscillation, rather than on the relatively flat steady-state region. If the repetition rate is changed or a pause is inserted between pulses, the intracavity energy may vary significantly. As a result, the energy of the next output pulse may be significantly affected by the time between pulses, which may be undesirable.
As a specific example, consider the case in which a user would like several trains of pulses at a high repetition rate, but with each train starting at a prescribed time determined by the user, so that the delay between the last pulse of the previous train and the first pulse of the upcoming train can vary train-to-train. For this case, the first pulse in each train may have a pulse energy that varies train-to-train, which is undesirable.
The undesirable variation in pulse energy is illustrated more fully in FIGS. 1 and 2, and the detailed explanation that follows.
Consider a continuously pumped, mode-locked, cavity-dumped laser 1 shown in FIG. 1. Two mirrors 4 and 9 define the laser cavity, which also typically contains a gain medium 6, a mode locker 5 and a controllable switch 2 (or “cavity-dumper”). The energy inside the laser cavity (or “intracavity energy”) takes the form of a pulse that bounces back and forth between the two mirrors. For each round-trip pass of the pulse in the cavity, the pulse passes twice through the gain medium 6. The pump for the gain medium 6 remains on throughout operation of the laser. For each pass of the pulse through the cavity-dumper 2, the cavity-dumper 2 can either allow the intracavity energy to remain inside the cavity, or direct the intracavity energy out of the cavity into an output beam 14. The cavity-dumper 2 typically has a polarizer 8 and an electro-optic modulator 7, which is driven electrically by a controller 3. The electro-optic modulator 7 rotates the plane of polarization of a transmitted beam, in response to the voltage produced by the controller 3. There is typically a photodiode 10 that receives a small amount of transmitted light through mirror 4 and generates a synchronization signal for the controller 3, so that the cavity dumper 2 may be switched at an appropriate point in the “bounce” of each pulse.
The laser 1 output can be switched “on” and “off” by the controller 3, where the “on” portion produces a stream of pulses, and the “off” portion produces no pulses. The controller is driven by a controller driving signal 11, a portion of which is shown in FIG. 1. The controller driving signal 11 has “on” portions, such as element 15, and “off” portions, such as element 16.
The controller 3 converts the controller driving signal to a modulator driving signal 12, which switches the electro-optic modulator 7 for a particular duration during each pulse round-trip in the cavity. During the “on” portion corresponding to each pulse, the switched electro-optic modulator 7 changes the polarization state of the traveling pulse so that it is reflected by polarizer 8 and directed out of the cavity into the output beam 14.
The intensity of the output beam contains streams of pulses 13 when the controller driving signal is “on”, and is effectively zero when the controller driving signal is “off”. The pulses are spaced apart in time by a multiple of the round-trip time of a pulse in the cavity.
Note that for the laser 1, the energy contained in the first pulse is not necessarily constant from train-to-train. For instance, the energy in pulse 17 is less than in pulse 19 but greater than in pulse 20. For this laser, the energy of the first pulse in the train depends on the length of the “off” portion that immediately precedes the train. In particular, the repetition rate of the laser 1 is fairly high, and is comparable to the relaxation rate of the cavity energy. This variation of the energy contained in the first pulse is undesirable, and the cause of this variation is explained in more detail in the paragraphs that follow.
FIG. 2 shows the energies in the cavity 60a and in the output beam 60b in greater detail, as a function of time. Prior to the time interval shown, there is a history of pulses, denoted by element 66, which is relatively unimportant for this discussion.
The history of pulses is long enough so that in region 67, the pulses have a relatively constant energy from pulse-to-pulse. This region is analogous to region 18 in FIG. 1. The pulse repetition rate is f, so that the pulse-to-pulse time is 1/f. The intracavity energy is Esteem, and the output energy is EConst.
Note that each pulse drains the cavity of a certain amount of energy, which is directed into the output beam and forms the pulse. After each pulse, the intracavity energy begins climbing again. The climbing and draining amounts are roughly equal, as long as the pulse repetition rate f remains roughly constant.
Region 68 is analogous to the “off” region between region 18 and pulse 19 in FIG. 1. In this region, the controller driving signal is set to “off”, and the modulator driving signal is set so that the polarization of the pulse transmits through the polarizer, and the light pulse remains in the cavity. In this laser, the pump is always on, so that energy continuously enters the laser cavity.
Note that the laser cavity includes both the gain medium and the intracavity energy. The intracavity energy oscillations seen in region 68 indicate that energy “sloshes back and forth” between the gain medium, where it is stored as a population inversion, and the intracavity energy, where it is stored in the electric field. The “sloshing” last for a few oscillations before the intracavity energy settles to a steady-state value, denoted by Esteady-state.
There is a characteristic time that describes how long the laser takes to settle to this steady-state value, which may be referred to as a “relaxation time” for the laser cavity. It is assumed for FIG. 2 that the duration of region 68 is comparable to the relaxation time, although this is not a requirement for the operation of the laser. Note that in general, this relaxation time is difficult to adjust in a laser.
Region 69 in FIG. 2 is analogous to the “on” region in FIG. 1 beginning with pulse 19. The controller driving signal 15 is set to “on”, the modulator driving signal switches the modulator once for each pulse, so that when switched, the beam reflects off the polarizer and is directed into the output beam. The pulses in the output beam form a train, with the amplitude of the first pulse being different from the amplitudes of the other pulses in the train.
Pulse 61 in FIG. 2 is analogous to pulse 19 in FIG. 1. A substantial portion of the initial intracavity energy, here being fairly close to Esteady-state, is directed into the output beam and forms pulse 61. It is seen that the energy contained in pulse 61 may vary significantly from the energy EConst of the pulses found in region 67. After this initial pulse 61, the intracavity energy begins climbing again, in a manner analogous to region 67 and to the leftmost portion of region 68 in FIG. 2. After a few pulses 62, 63 and 64, the pulse energy returns to EConst and the intracavity energy returns to Estream, as in region 67.
A drawback to the laser of FIG. 1, with the energy profiles of FIG. 2, is that the first pulse 61 in a train has an energy that may be different from the energies of subsequent pulses 62-65 in the train. And worse, the energy of the first pulse 61 may depend on the length of the “off” region 68 that immediately precedes it. This is undesirable.
Accordingly, there exists a need for a mode-locked, cavity-dumped laser with a high repetition rate, in which the energy of the first pulse of a train is independent of the time between pulse trains, and is essentially equal to the energies of subsequent pulses in the train.